Immunocytochemistry has been a powerful tool in research in cell biology. It allows the precise localization of individual proteins to cell organelles and -- when carried to the electron microscope level -- even to specific subcompartments thereof. It provides information on localization of proteins in intact cells and therefore is essential in that it complements cells fractionation and biochemical approaches. To apply immunocytochemical methods at the electron microscope level is technically demanding and requires expensive instrumentation and highly skilled and experienced personnel. The purpose of Core B is to provide this expertise and make it possible for all members of the Program Project to apply these techniques in their program-related research. Personnel associated with the Core have considerable experience in all major approaches currently used in immunocytochemistry involving immunofluorescence, immunoperoxidase techniques, and immunogold labeling of plastic embedded (Lowacryl or LR White) or ultrathin cryosections. The Core Director has used these techniques in her research over 15 years, and Mr. McCaffery, the Core's Technical Director, has had over 10 years experience in this area and has coauthorized a number of publications using these techniques. In addition, we have enlisted the aid of K.T. Tokuyasu, the originator of most of the techniques used in ultrathin cryolabeling, as a consult to the Core. We intend to operate the facility as a state-of-the-art Core to serve program members.